The present invention relates to a high withstand voltage p-channel MOSFET using, as a semiconductor crystal, silicon carbide which is a kind of wide band gap (hereinafter abbreviated to as WBG) crystalline material.
FIG. 1 shows a circuit of a three-phase inverter IGBT module which is a representative power semiconductor module. In this inverter circuit, three arms each composed of series-connected two units are provided in parallel with one another on the assumption that a circuit in which a cathode and an anode of a free wheeling diode (hereinafter abbreviated to as FWD) are back-to-back connected between a collector and an emitter of an IGBT is regarded as one unit. The three arms are connected to terminals P and N which are common to the three arms and between which a main current flows. Junctions between the units in the three arms are provided as output terminals U, V and W respectively. To drive the IGBT module forming such an inverter circuit, a lower arm element gate driving circuit for driving the lower IGBT's of the arms is connected to three upper arm element gate driving circuits for driving the upper IGBT's of the arms respectively. These driving circuits are further connected to a control circuit while electrically insulated by photo couplers not shown, respectively. The gate driving circuits require forward bias and backward bias power sources not shown.
When the IGBT module shown in FIG. 1 is driven, one lower arm power source and three upper arm power sources, that is, four power sources in total are required as the power sources. As a result, the configuration of the gate driving circuits is complicated causing an increase in device size and a resulting increase in cost. As described above, increase in size of the gate driving circuits is caused by use of all homopolarity n-channel IGBT's as IGBT's mounted in the IGBT module. There is always a demand for reduction in cost from the market.
It is known well that the number of power sources necessary for the aforementioned gate driving circuits can be reduced to eliminate the increase in cost if the IGBT module is replaced by a complementary IGBT module composed of n-channel IGBT's and p-channel IGBT's of reverse polarity (JP-A-63-253720 and JP-A-2001-85612). For example, as shown in FIG. 2, the complementary IGBT module has a circuit configuration in which p-channel IGBT's are arranged in lower arms, n-channel IGBT's are arranged in upper arms, and respective FWD's are back-to-back arranged in the same manner as shown in FIG. 1. According to the circuit configuration, the number of gate driving circuits can be reduced from 4 to 3 so that reduction in device size and, accordingly, reduction in cost can be expected. Although an inverter is specified so that a dead time of several μsec long is set in on/off switching timing to avoid short-circuiting caused by simultaneous switching-on of the upper and lower arms, the dead time can be shortened according to the complementary module of the circuits shown in FIG. 2. As a result, there is a merit that distortion of output waveforms can be reduced.
The aforementioned complementary IGBT module is however currently unavailable on the market. This is because breakdown tolerance of p-channel IGBT's is too low to endure practical use compared with that of n-channel IGBT's. IEEE Transaction on Electron Devices (Vol. 38, pp. 303-309, 1991) has also described that p-channel IGBT's have low avalanche breakdown tolerance theoretically on operation. On the other hand, a silicon semiconductor (hereinafter abbreviated to as Si) is currently generally used as a substrate material of a power semiconductor element for controlling a high withstand voltage and a large current. On the market, there is a strong demand for a power device satisfying both a large current and a high speed. For this reason, stronger emphasis has been placed on improvement of IGBT's and power MOSFET's, so that the improvement has advanced up to an almost limit of the Si substrate material at present. Therefore, other semiconductor crystalline materials have been discussed from the viewpoint of improvement in large current and high speed of the power semiconductor element. As described in IEEE Transaction on Electron Devices (Vol. 36, p. 1811, 1989), silicon carbide semiconductor (hereinafter abbreviated to as SiC) is particularly excellent in low on-voltage and high speed/high temperature characteristic, so that attention is drawn to SiC as a next-generation power semiconductor material. Moreover, SiC is a chemically super-stable material which has a wide band gap of 3 eV and which is so excellent that SiC can be used extremely stably as a semiconductor even at a high temperature.
In addition, attention is drawn to a power MOSFET as a power semiconductor element using the SiC as a semiconductor substrate. FIG. 14 is a sectional view showing important part of a general n-channel trench MOSFET made of SiC. The SiC n-channel trench MOSFET has an n+ drain substrate (referred to as n++-SiC substrate in FIG. 14) 20, an n-type drift layer (referred to as n−-SiC layer 31 in FIG. 14) 31, a p base layer (referred to as p-SiC layer 23 in FIG. 14) 23, and an n+ source region (referred to as n+-SiC layer 24 in FIG. 14) 24 selectively formed on a surface layer of the p base layer 23. A trench 25 is provided so as to extend from a front surface of the n+ source region 24, pass through the p base layer 23 and reach the n-type drift layer 31. The trench 25 is further filled with a gate electrode 27 made of conductive polysilicon or the like through a gate insulating film 26 formed on an inner surface of the trench 25. The symbols “+” and “−” attached to the right shoulders of the characters “n” and “p” show a relatively high impurity concentration and a relatively low impurity concentration respectively.
It is however known that a p-channel MOSFET obtained by inverting the polarity in the layer structure of the aforementioned SiC n-channel trench MOSFET exhibits such characteristic that an avalanche ionization rate for holes takes a larger value than an avalanche ionization rate for electrons under the condition that the same electric field is applied (Material Science Forum 2004, pp. 673-676, 2004). What is meant by this is that the p-channel MOSFET using holes as a carrier of a main current generates avalanche more frequently than the n-channel MOSFET using electrons as a current carrier. As a result, even in SiC MOSFET's, like the aforementioned SiC IGBT's, the p-channel MOSFET has lower avalanche breakdown tolerance than the n-channel MOSFET. Accordingly, SiC p-channel MOSFET's of practical use as well as IGBT's are unavailable on the market.
Moreover, in the n-channel trench MOSFET produced with use of a WBG substrate as shown in FIG. 14, because the maximum electric field intensity of the WBG is stronger than that of Si as described above, dielectric breakdown of the silicon oxide film is caused by an electric field applied to the bottom portion of each gate trench before the WBG reaches the avalanche breakdown electric field when a high voltage is applied between the source and drain of the MOSFET. To suppress the dielectric breakdown, a p+-type region is provided in the bottom portion of each gate trench in the SiC n-channel trench MOSFET to thereby prevent application of an electric field not lower than the allowable electric field of the gate oxide film (IEEE Transaction on Electron Devices (Vol. 36, p. 1811, 1989)). The same thing can apply to the p-channel MOSFET. The provision of a dielectric breakdown protecting region in the bottom portion of each gate trench as described above is such hard work that cannot be said to be easy production because the p-channel MOSFET is likely to suffer not only increase of superfluous processes but also increase of on-resistance if the protecting region is not provided with good positional accuracy, like the n-channel MOSFET. If possible, it is desired to avoid the process of forming the protecting region.
Great expectations will be however placed on the future extension of SiC for use in power semiconductor devices, especially as an MOSFET semiconductor material because there is a high possibility that SiC will overcome the material limit in semiconductor characteristic of Si. If a p-channel MOSFET highly useful in practical use can be made of SiC, the merit thereof is large because a complementary MOSFET module highly useful in practical use can be formed when the p-channel MOSFET is combined with an n-channel MOSFET.